1. Field of the Invention
The present invention relates to disc refining of wood pulp as a papermaking stock preparation process. More specifically, the present invention relates to the bar and channel pattern in the refiner section of a disc refiner plate.
2. Description of the Prior Art
In the preparation of wood pulp for papermaking, cellulosic fibers are subjected to a series of mechanical working or shearing operations prior to final web forming. The generally concluded objective of such mechanical stock working is to develop increased fibrillation of individual cellulose fibers which contribute to increased paper web strength. This process is characterized by the papermaking industry as "refining " and the machines for performing the process as refiners.
Although there are several design types of refiners available to the industry, the type to which the present invention is directed is the "plate" or "disc" refiner. The construction and operation of disc refiners is described more fully in U.S. Pat. Nos. 2,654,295 and 2,778,282 to L. M. Sutherland and U.S. Pat. No. 2,651,976 to D. M. Sutherland.
Generally, disc refiners comprise a pair of disc shaped grinding elements disposed for opposite hand relative rotation, usually about a common axis that is normal to the disc plane. The rotational planes of such discs are parallel with an adjustable space therebetween. Wood pulp stock flow is introduced as an aqueous slurry at the disc center and progresses through a fluid shear region between the disc faces toward the outer periphery thereof.
Prior art design of such refiner discs normally specifies a serrated surface comprising an alternating series of ridges and channels extending in generally radial directions. Such ridges are characterized as "bars."
Design modifications to the prior art specify parallelism of adjacent bars over a small arcuate section of the disc. Since a large fraction of the bars in a section will be of truncated length, large radial feed grooves are provided as stock conduits to the channels between the truncated bars. As a consequence of such flow patterns, however, all the stock delivered to a refiner is not subjected to equal mechanical treatment.
Ideally, each fiber should receive the same number of shear events which occur between passing bars respective to each disc. However, refining efficiency relates to the number of such shear events per relative revolution of a disc pair. Consequently, it is advantageous to provide as many bars and companion channels as the structural integrity of the disc material will permit. Normally, therefore, the bars may be as narrow as one eighth inch (0.32 cm) spaced by an equal width channel.
Another consideration is that due to the circular geometry of the flow region, the refining area of a disc pair increases by the square of the radius.
Collectively, therefore, a design compromise is forced between maximum refining efficiency for each annular increment of disc area and uniform mechanical treatment of each pulp fiber.
The prior art has promulgated numerous design solutions for the stated compromise. Those designs disclosed by the Sutherland patents, supra, are typical.
In a related art, that of grain grinding, a slightly different approach to disc design has been suggested. This approach generally comprises a curved, rather than straight line, path of the grain outward to the disc periphery. Such curved bar patterns are disclosed in U.S. Pat. Nos. 19,273; 27,551; 71,733; 120,505, 348,637; 499,714; 1,609,717; and 1,705,379. However, we are aware of only one instance, that disclosed by R. L. Reinhall in U.S. Pat. No. 3,674,217, in which a curved bar pattern has been suggested for use on wood pulp. Even in the case of Reinhall, the teaching is applied to defibration which is a gross separation of wood fiber bundles form a larger, chip particle, conglomerate. While the mechanical processes are outwardly similar, the comparison of defibration to refining is akin to that drawn between a blacksmith and a silversmith.
Nevertheless, the curved bar technique of disc design has been found to offer significant advantages in refiner efficiency and uniform stock treatment. However, due to the relative complexity of curved bar patterns, it would be questionable that such advantages outweigh the consequent cost burden.
It is, therefore, an object of this invention to teach a refiner plate design which provides uniform mechanical treatment to each fiber element passing therethrough.
Another object of the present invention is to teach a refiner disc bar pattern that is highly efficient in providing the maximum number of fiber impact events possible per corresponding radial increment.
Another object of the present invention is to teach a curved bar pattern for refiner discs that provides a uniform and continuous width flow channel between all bars from the inner ring groove to the outer, disc periphery.
Another object of the present invention is to teach a curved bar pattern for refiner discs that provides a substantially uniform bar thickness for all bars from the inner ring groove to the outer disc periphery.
Another object of the present invention is to teach a curved bar pattern having curved bar channels that may be conveniently machined with conventional machine tools on relatively simple jigs and fixtures.